Added face crop examples in the doc

bob/bio/face/config/baseline/helpers.py

@@ -252,12 +252,12 @@ def get_default_cropped_positions(mode, cropped_image_size, annotation_type):
     """
     if mode == "legacy":
         return legacy_default_cropping(cropped_image_size, annotation_type)
-    elif mode in ["facenet", "arcface"]:
+    elif mode in ["dnn", "facenet", "arcface"]:
         return dnn_default_cropping(cropped_image_size, annotation_type)
     elif mode == "pad":
         return pad_default_cropping(cropped_image_size, annotation_type)
     else:
-        raise ValueError("Unknown default cropping mode {}".format(mode))
+        raise ValueError("Unknown default cropping mode `{}`".format(mode))
 
 
 def make_cropper(

doc/faq.rst

@@ -11,4 +11,63 @@ The recipe below helps you to set a face cropper based on eye positions.
 
 .. literalinclude:: faq/facecrop.py
 
-.. figure:: ./faq/img/ada_cropped.png
\ No newline at end of file
+.. figure:: ./faq/img/ada_cropped.png
+
+
+How to choose the cropped positions ?
+=================================
+
+The ideal cropped positions are dependent on the specific application you are using the face cropper in.
+Some face embedding extractors work well on loosely cropped faces, while others require the face to be tightly cropped.
+We provide a few reasonable defaults that are used in our implemented baselines. They are accessible through an utilitary function,
+:any:`bob.bio.face.config.baseline.helpers.get_default_cropped_positions`.
+::
+
+    import bob.bio.face.config.baseline.helpers.get_default_cropped_positions
+    mode = 'legacy'
+    cropped_image_size=(160, 160)
+    annotation_type='eyes-center'
+    cropped_positions = get_default_cropped_positions(mode, cropped_image_size, annotation_type)
+
+
+There are currently three available modes :
+
+* :code:`legacy` Tight crop, used in non neural-net baselines such as :code:`gabor-graph`, :code:`lgbphs` or :code:`lda`. 
+  It is typically use with a 5:4 aspect ratio for the :code:`cropped_image_size`
+* :code:`dnn` Loose crop, used for neural-net baselines such as the ArcFace or FaceNet models.
+* :code:`pad` Tight crop used in some PAD baselines in :ref:`bob.pad.face`
+
+We present hereafter a visual example of those crops for the `eyes-center` annotation type.
+
+.. figure:: img/cropping_example_source.png
+    :height: 250px
+    :align: left
+    :alt: Source image
+    :figclass: align-center
+
+    Original face image
+
+.. figure:: img/cropping_example_legacy.png
+    :height: 250px
+    :align: right
+    :alt: Legacy crop
+    :figclass: align-center
+
+    Legacy crop (160 x 128)
+
+.. figure:: img/cropping_example_dnn.png
+    :height: 250px
+    :align: left
+    :alt: DNN crop
+    :figclass: align-center
+
+    DNN crop (160 x 160)
+
+.. figure:: img/cropping_example_pad.png
+    :height: 250px
+    :align: right
+    :alt: PAD crop
+    :figclass: align-center
+    
+    PAD crop (160 x 160)
+    
\ No newline at end of file